Image-forming MR methods, which utilize the interaction between magnetic field and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, they do not require ionizing radiation, and they are usually not invasive.
According to the MR method in general, the body of a patient or in general an object to be examined is arranged in a strong, uniform magnetic field B0 whose direction at the same time defines an axis, normally the z-axis, of the coordinate system on which the measurement is based.
The magnetic field produces different energy levels for the individual nuclear spins in dependence on the applied magnetic field strength which spins can be excited (spin resonance) by application of an alternating electromagnetic field (RF field) of defined frequency, the so called Larmor frequency or MR frequency. From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field extends perpendicularly to the z-axis, so that the magnetization performs a precessional motion about the z-axis.
Any variation of the magnetization can be detected by means of receiving RF antennas, which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicularly to the z-axis.
In order to realize spatial resolution in the body, switching magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving antennas then contains components of different frequencies which can be associated with different locations in the body.
The signal data obtained via the receiving antennas corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of samples of k-space data is converted to an MR image, e.g. by means of Fourier transformation.
One important aspect in magnetic resonance imaging is effective shielding using RF (radio frequency) shields (or RF screens) to screen the above mentioned RF fields towards the gradient coils. There are two major requirements regarding the design of RF-screens: the first requirement is that the RF-screen must be well conducting to act as a shield for the radio frequency fields in the MHz-range generated by the RF antenna of the MR system. However, a favorable highly conducting RF-screen which effectively blocks the RF signals of the RF antenna acts at the same time as a medium in which Eddy currents can be induced due to the switching magnetic field gradients in the kHz-range, wherein these Eddy currents cause substantial heat dissipation in the RF-screen. Thus, the second requirement is that the RF-screen is able to effectively suppress the development of Eddy currents.
However, good RF-screening requires highly conductive materials, whereas highly conductive materials lead to the development of unwanted Eddy currents.
In order to solve this conflict typically slitted RF-screens are used, wherein the slits of the RF-screen are bridged by capacitors to ‘close’ them for RF. These capacitors can either be discreet components or they may also be distributed capacitors.
For example U.S. Pat. No. 7,230,427 B2 discloses a magnetic resonance apparatus with an RF antenna unit and a gradient coil unit, wherein the MR apparatus comprises an RF shield disposed between the RF antenna unit and the gradient coil unit.
Even though slitting RF-screens and bridging the resulting slits by capacitors yields a good compromise between high conductance for RF shielding and Eddy current suppression, RF-screens manufactured in this manner are expensive due to the slitting and bridging procedure.
From the foregoing it is readily appreciated that there is a need for an improved RF shield. It is consequently an object of the invention to provide an improved magnetic resonance imaging system comprising an improved RF shield.